Biomechanical evaluation of stability of lumbosacrumal spine following resection of nucleus pulposus of intervertebral disc in vitro

Objective:The stability of lumbosacrumal spine was evaluated immediately after resection of nucleus pulposus of intervertebral disc(RNP).Methods:The ranges of motions,compression rigidity and shearing rigidity were analyzed before and after RNP at L 4 L 5 and L 5 S 1 levels respectively on six adult fresh human cadavers.Motions of flexion/extension,lateral bending and axial rotation were produced by six pure moments with maximum of 10.0N·m and measured with stereophotogrammetry.The compression rigidity and shearing rigidities in anterioposterior and lateral were measured under 1000N and 600N respectively.Results:The segmental motions in all directions increased after RNP.The motions increased significantly in extension and left/right axial rotation at L 4 L 5 level,and very significantly in flexion and significantly in left/right axial rotation at L 5 S 1 level.The compression rigidity and anteroposterior shearing righidity lowered and lateral shearing righidity lowered very significantly after RNP.Conclusions:The lumbosacrumal spine following RNP becomes instability because segmental motions increase and rigidities in compression and shearing lower,which increases loads on the posterior structures of spine and accelerates retrograde degeneration of lumbosacrumal spine.